A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The rate at which the substrates can be patterned, known as throughput, is a major performance criterion in lithography apparatus systems and it is beneficial to increase this rate. Throughput is dependent on multiple factors, for example the speed at which exposure takes place and the speed at which measurements can be taken. Therefore, it is beneficial to have high scanning speeds for both exposure and measurement. However, it is important to maintain accuracy of measurements and patterning at the high scanning speeds.
Radiation beams in atmospheric systems pass through gas, which occurs for example, in a lithographic apparatus. Local fluctuations in the characteristics of a gas can affect a radiation beam passing through it. In known apparatus, barriers are used to reduce the effects of fluctuations in gases on radiation beams. However, known barriers become less effective at high speeds. Therefore, it is an aim of the present invention to provide an apparatus with an improved barrier system.